The present invention relates to a method and/or architecture for implementing devices for generating precision currents generally and, more particularly, to a method and/or architecture for rapidly switching a precision current on and off.
Various systems (i.e., high speed D/A converters) switch large, precise currents to establish signaling. However, such large, precise current driver cells are difficult to quickly power (i.e., turn on and off). The driver cells need delay time for bias voltages and output currents to settle.
Referring to FIG. 1, a typical large, precise current switch 10 is shown. The current switch 10 implements the high speed mode (i.e., 480 Mb/s) described in the Universal Serial Bus (USB) Specification, Version 2.0, published April 2000 and hereby incorporated by reference in its entirety. The USB 2.0 specification defines an output made by switching a nominal current of 18 mA into a 22.5 ohm load to provide 400 mV voltage swings.
The circuit 10 includes a reference circuit 12 configured to establish the bias voltage PBIAS and a current output circuit 14 configured to generate the 18 mA current and differential output signal. The reference circuit 12 includes a voltage bandgap generator 16 and a reference amplifier 18 configured to generate the voltage PBIAS. The voltage PBIAS is applied to a PMOS output transistor Q1 of the output circuit 14 to generate the desired output current IOUT (i.e., the 18 mA current). During transmission, the current IOUT is switched between the differential outputs DP and DM, as selected by the signal OUTPUT_DATA to generate 0 mV and 400 mV voltages. During non-transmission, the output enable signal OUTPUT_EN switches the current IOUT to an internal ground node (i.e., a dump path), where the output is disabled. Therefore, the 18 mA current is not output to the differential signals DP and DM and the lines are at 0 mV.
Once the current IOUT is turned on (i.e., IOUT is switched from 0 mA to 18 mA and flows to either the internal dump path or the outputs), a voltage swing at the node DRAIN couples a disturbance into the node PBIAS. The disturbance on the node PBIAS needs time to settle out. The settling time depends on the size of transistor Q1 and the characteristics of the reference amplifier 18. To avoid the setting time delay, the current IOUT is continuously enabled such that a device can start transmission whenever necessary. The configuration causes increased power dissipation due to the current IOUT being directed to the dump path during non-transmit times.
There are two main power issues relating to the high-speed mode current for USB. First, in USB hubs there are at least 2 and as many as 8 ports that each need a precision 18 mA current during transmission. The power dissipation during non-transmit time is large, since the 18 mA current IOUT is continuously enabled. Secondly, for high-speed (i.e., non-hub) peripherals, there is a competitive advantage to having low enough power consumption to qualify as a USB low power device. USB low power devices are limited to 100 mA current consumption from a USB power source (i.e., from the upstream hub or host). The USB low power rating simplifies powering the downstream devices from a bus-powered (i.e., not self-powered) hub and provides additional flexibility to the USB network.
Settling problems prohibit the circuit 10 from turning on the current IOUT just before transmitting and turning off the current IOUT just after transmitting. For example, USB 2.0 devices have a lead time (i.e., the time before a transmit, once a device recognizes it is to transmit) that is much smaller than the time normally required for the circuit 10 to settle to the desired accuracy. Rapidly turning on the current IOUT in the transistor Q1 (i.e., by closing a switch to the output) causes the bias voltage PBIAS to be disturbed through coupling. The circuit 10 results in an inaccurate current until the bias voltage PBIAS settles at the desired value.
It is generally desirable to provide a device configured to perform rapid switching of a precise current.
The present invention concerns an apparatus comprising a reference circuit, a correction circuit and an output circuit. The reference circuit may be configured to generate a bias signal. The correction circuit may be configured to correct a bias voltage of the bias signal. The output circuit may be configured to generate an output current in response to the bias signal. The bias signal may be corrected in response to the bias voltage.
The objects, features and advantages of the present invention include providing a method and/or architecture for rapidly switching a precision current on and off that may (i) lower overall power consumption, (ii) be implemented without a wide-bandwidth amplifier to set the bias voltage, (iii) provide a stable amplifier configuration to set references, (iv) isolate the wideband function in a correction amplifier, and/or (v) enable the correction amplifier in a small amount of time.